There is disclosed in my copending application, Ser. No. 358,411, filed May 8, 1973, a method and system for the catalytically supported thermal combustion of carbonaceous fuels. The invention of said application pertains particularly to the catalytically supported thermal combustion of carbonaceous fuels under essentially adiabatic conditions, without the formation of substantial emissions of carbon monoxide and nitrogen oxides (NO.sub.x), particularly the latter.
Disclosed in my copending application is a method of combustion which is distinct from purely thermal combustion and purely catalytic combustion. The method comprises a unique combination of catalytically supported thermal combustion.
In conventional thermal combustion fuel and air in inflammable proportions are contacted with an ignition source, e.g., a spark, to ignite the mixture which will then continue to burn. Flammable mixtures of most fuels are normally burned at relatively high temperatures, i.e., in the order of about 3300.degree.F. and above, which inherently results in the formation of substantial emissions of NO.sub.x. In the case of gas turbine combustors, the formation of NO.sub.x can be greatly reduced by limiting the residence time of the combustion products in the combustion zone. However, even under these circumstances undesirable quantities of NO.sub.x are nevertheless produced. In addition limiting such residence time makes it difficult to maintain stable combustion even after ignition.
In purely catalytic combustion systems, there is little or no NO.sub.x formed in a system which burns the fuel at relatively low temperatures. Catalytic combustion heretofore has been generally regarded as having limited practicality in providing a source of power as a consequence of the need to employ impractically large amounts of catalyst so as to make a system unduly large and cumbersome. Consequently, catalytic combustion has been limited generally to such operations as treating tail gas streams of nitric acid plants, where the catalytic reaction is employed to heat spent process air containing about 2% oxygen at temperatures in the range of about 1400.degree.F. Catalytic oxidation reactions follow the course of the graph of FIG. 1 of the accompanying drawings, to the extent of Regions A, B and C of that graph. FIG. 1 represents a plot of temperature against rate of reaction.
For any given catalyst and set of reaction conditions, as the temperature in catalytic combustion is initially increased, the reaction rate is also increased as shown in the kinetic region A of the rate curve of FIG. 1. This rate of increase is exponential with temperature. As the temperature is raised further, the reaction rate then passes through a transistion zone where the limiting parameters determining reaction rate shift from catalytic to mass transfer (region B of the curve in FIG. 1). When the catalytic rate increases to such an extent that the reactants cannot be transferred to the catalytic surface fast enough to keep up with the catalytic reaction rate, the reaction shifts to mass transfer control, and the catalytic reaction rate levels off regardless of further temperature increases. The reaction is then said to be mass transfer limited (region C of the curve of FIG. 1). In mass transfer controlled catalytic reactions, one cannot distinguish between a more active catalyst and a less active catalyst because the intrinsic catalyst activity is not determinative of the rate of reaction. Regardless of any increase in catalytic activity above that required for mass transfer control, a greater catalytic conversion rate cannot be achieved for the same set of conditions.
Because of this limitation, in order to increase the conversion rate for any given system, it appears essential either to increase the amount of catalyst surface or to increase the rate of mass transfer of reactants to the surface. The former, for practical combustion systems, would require either a catalyst size of such magnitude as to be unwieldy or a catalyst configuration which results in increased specific pressure drop and which would require unwieldy geometry to hold a total pressure drop constant. For example, in the case of gas turbine engines, the catalytic reactor might very well be larger than the engine itself. On the other hand, increasing the rate of mass transfer of reactants to the catalytic surfaces would result in increased pressure drop and consequently a substantial loss of energy; sufficient pressure drop may not even be available to provide the desired rate of reaction. Quite obviously, these approaches, while theoretically possible, are quite impractical.
The method and system of my said copending application stems from my discovery that it is possible to achieve essentially adiabatic combustion in the presence of a catalyst at a reaction rate many times greater than the mass transfer limited rate. That is, I have found that catalytically-supported, thermal combustion surmounts the mass transfer limitation. If the operating temperature of the catalyst is increased substantially into the mass transfer limited region, the reaction rate again begins to increase exponentially with temperature (region D of the curve of FIG. 1). This is an apparent contradiction of catalytic technology and the laws of mass transfer kinetics. The phenomena may be explained by the fact that the catalyst surface and the gas layer near the catalyst surface are above a temperature at which thermal combustion occurs at a rate higher than the catalytic rate, and the temperature of the catalyst surface is above the instantaneous autoignition temperature of the fuel-air admixture (defined hereinbelow). The fuel molecules entering this layer spontaneously burn without transport to the catalyst surface. As combustion progresses, it is believed that the layer becomes deeper. The total gas is ultimately raised to a temperature at which thermal reactions occur in the entire gas stream rather than only near the surface of the catalyst. Once this stage is reached within the catalyst, the thermal reactions continue even without further contact of the gas with the catalyst as the gas passes through the combustion zone.
The term "instantaneous auto-ignition temperature" for a fuel-air admixture as used herein is defined to mean that the temperature at which the ignition lag of the fuel-air mixture entering the catalyst is negligible relative to the residence time in the combustion zone of the mixture undergoing combustion.
In accordance with the invention of my said copending application, catalytically-supported thermal combustion is achieved by contacting at least a portion of the carbonaceous fuel intimately admixed with air with a solid oxidation catalyst having an operating temperature substantially above the instantaneous auto-ignition temperature of the fuel-air admixture. At least a portion of the fuel is combusted under essentially adiabatic conditions. Combustion is characterized by the use of a fuel-air admixture having an adiabatic flame temperature substantially above the instantaneous auto-ignition temperature of the admixture but below a temperature that would result in any substantial formation of oxides of nitrogen. The adiabatic flame temperature is determined at catalyst inlet conditions. The resulting effluent is characterized by high thermal energy useful for generating power and by low amounts of atmospheric pollutants. Where desired, combustible fuel components, e.g., uncombusted fuel or intermediate combustion products contained in the effluent from the catalytic zone, or fuel-air admixture which has not passed through a catalytic zone, may be combusted in a thermal zone following the catalytic zone, as explained hereinbelow in greater detail.
Sustained catalytically-supported, thermal combustion of this invention occurs at a substantially lower temperature than in conventional adiabatic thermal combustion and therefore it is possible to operate without formation of significant amounts of NO.sub.x. Combustion is no longer limited by mass transfer as in the case of conventional catalytic combustion, and at the specified operating temperatures the reaction rate is substantially increased beyond the mass tranfer limitation, e.g., at least about 5 or 10 times greater than the mass transfer limited rate. Reaction rates of up to about 100 or more times the mass transfer limited rate may be attainable. Such high reaction rates permit high fuel space velocities which normally are not obtainable in catalytic reactions. I can employ, for instance, at least an amount of fuel equivalent in heating value to about 300 pounds of propane per hour per cubic foot of catalyst, and this amount may be at least several times greater, for instance, an amount of fuel equivalent in heating value to at least about 1000 pounds of propane per hour per cubic foot of catalyst. There is, moreover, no necessity of maintaining fuel-to-air ratios in the flammable range, and consequently loss of combustion (flame-out) due to variations in the fuel-to-air ratio is not the problem it is in conventional combustors.
The adiabatic flame temperature of fuel-air admixtures at any set of conditions (e.g., initial temperature and, to a lesser extent, pressure) is established by the ratio of fuel to air. The admixtures utilized are generally within the inflammable range or are fuel-lean outside of the imflammable range, but there may be instances of a fuel-air admixture having no clearly defined inflammable range but nevertheless having a theoretical adiabatic flame temperature within the operating conditions of the invention. The proportions of the fuel and air charged to the combustion zone are typically such that there is a stoichiometric excess of oxygen based on complete conversion of the fuel to carbon dioxide and water. Preferably, the free oxygen content is at least about 1.5 times the stoichiometric amount needed for complete combustion of the fuel. Although the invention is described herein with particularity to air as the non-fuel component, it is well understood that oxygen is the required element to support proper combustion. Where desired, the oxygen content of non-fuel component can be varied and the term "air" is used herein to refer to the non-fuel components of the admixtures. The fuel-air admixture fed to the combustion zone may have as low as 10 percent free oxygen by volume or less, which may occur, for example, upon utilization as a source of oxygen of a waste stream wherein the portion of this oxygen has been reacted. In turbine operations, the weight ratio of air to fuel charged to the combustion system is often above about 30:1 and some turbines are designed for air-to-fuel ratios of up about 100 or 200 or more:1.
The carbonaceous fuel, which when burned with a stoichiometric amount of air (atmospheric composition) at the combustor inlet temperature usually has an adiabatic flame temperature of at least about 3300.degree.F., is combusted essentially adiabatically in the catalyst zone. Although the instantaneous auto-ignition temperature of a typical fuel may be below about 2000.degree.F., stable adiabatic combustion of the fuel below about 3300.degree.F. is extremely difficult to achieve in practical primary combustion systems. It is for this reason that even with gas turbines limited to operating temperatures of 2000.degree.F., the primary combustion is typically at temperatures in excess of 4000.degree.F. As stated above, in the invention of my said copending application, combustion is characterized by using a fuel-air admixture having an adiabatic flame temperature substantially above the instantaneous auto-ignition temperature of the admixture but below a temperature that would result in any substantial formation of NO.sub.x. The selection of the adiabatic flame temperature limits is governed largely by residence time and pressure. Generally, adiabatic flame temperatures of the admixtures are in the range of about 1700.degree. to 3200.degree.F., and preferably are about 2000.degree. to 3000.degree.F. Operating at a temperature much in excess of 3200.degree.F. results in significant formation of NO.sub.x even at short contact times; this derogates from the advantages of the invention vis-a-vis a conventional thermal system. A higher temperature within the defined range is desirable, however, because the system will require less catalyst and thermal reactions are an order of magnitude or more faster, but the adiabatic flame temperature employed can depend on such factors as the desired composition of the effluent and the overall design of the system.
It will thus be observed that a fuel which would ordinarily burn at such a high temperature as to form NO.sub.x, is successfully combusted within the defined temperature range without significant formation of NO.sub.x. Although combustion occurs adiabatically, it should be understood that for practical operations there may be heat losses to the environment from the combustion zone. A loss in temperature as measured by the effluent temperature may be as much as about 300.degree.F. and preferably is not more than about 150.degree.F. Notwithstanding these minor heat losses, the operation from a practical standpoint is considered adiabatic, and the heat of reaction is released primarily in the effluent gases. Thus there may be about four times, preferably at least about seven times, more heat released (thermal energy) in these gases than is lost from the combustion zone.
The catalyst in the catalytically supported thermal combustion generally operates at a temperature approximating the theoretical adiabatic flame temperature of the fuelair admixture charged to the combustion zone. The entire catalyst may not be at these temperatures, but preferably a major portion, or essentially all, of the catalyst surface is at such operating temperatures. These temperatures are usually in the range of about 1700.degree. to 3200.degree.F., preferably about 2000.degree.F. to about 3000.degree.F. The temperature of the catalyst zone is controlled by controlling the composition and initial temperature of the fuel-air admixture, i.e., adiabatic flame temperature, as well as the uniformity of the mixture. Relatively higher energy fuels can be admixed with larger amounts of air in order to maintain the desired temperature in a combustion zone. At the higher end of the temperature range, shorter residence times of the gas in the combustion zone appear to be desirable in order to lessen the chance of forming NO.sub.x. The residence time is governed largely by temperature, pressure and space throughput, and generally is measured in milliseconds. The residence time of the gases in the catalytic combustion zone and any subsequent thermal combustion zone may be below about 0.1 second, preferably below about 0.05 second. The gas space velocity may often be, for example, in the range of about 0.5 to 10 or more million cubic feet of total gas (standard temperature and pressure) per cubic foot of total combustion zone per hour. For a stationary turbine burning diesel fuel, typical residence times could be about 30 milliseconds or less; whereas in an automotive turbine engine burning gasoline, the typical residence time may be about 5 milliseconds or less. The total residence time in the combustion system should be sufficient to provide essentially complete combustion of the fuel, but not so long as to result in the formation of NO.sub.x.
Nitrogen oxides found in the effluent may have been introduced to the system from the air supply or even from the fuel as an impurity. Combustion occurs, however, without the substantial formation of NO.sub.x, in my catalytically supported thermal combustion system. Typically, the combustion effluent will contain less than about 15, or less than about 10, parts per million by volume of NO.sub.x above the amount fed to the combustion system. Values lower than those present in the incoming air have been measured. In addition, the effluent from combustion of a nitrogen free carbonaceous fuel may typically contain less than about 2 parts per million by volume of NO.sub.x. It is of further significance that the effluent typically may contain less than about 10 parts per million by volume hydrocarbons, and frequently even less than about 300 parts per million by volume carbon monoxide, and even less than about 20 parts per million. Effluents this low in pollutants are most acceptable and are far below any requirements of the Federal Emission Standards established by the Environmental Protection Agency for 1976 for automobile emissions.
The present invention, as in accordance with my earlier co-pending application Ser. No. 197,323, filed Nov. 10, 1971, provides a gas turbine system exhibiting markedly increased efficiency and greater power and employing catalytically supported thermal combustion in accordance with my earlier application Ser. No. 358,411. According to the present invention, combustion occurs under essentially adiabatic conditions to form an effluent containing uncombusted fuel values, and at least part of the uncombusted fuel values are thermally combusted in an expansion zone positioned in the path of the effluent to counteract the cooling effect occurring on expansion of the gases within the gas turbine. The turbine expansion zone is placed sufficiently close to the catalyst zone such that thermal combustion occurring subsequent to the catalyst will not go to completion except in the expansion zone. The turbine expansion zone, for example, may be the turbine wheel itself or nozzle means prior to the turbine wheel, or any combination thereof, and the like. In this manner, it is possible to maintain the temperature of the expanding gases at a higher level than otherwise obtainable. Thus, it is possible to obtain more power for a given size turbine, and to obtain higher efficiency in operation.